Production of magnesia



Patented June 9, 1953 PnonUcrIoN MAGNESIA Leslie W. Austin, San Jose, Califi, assignor to Kaiser Aluminum & Chemical (Corporation, Oakland, Calif., a corporation of Delaware No. Drawing. Application May 17,. 1950,,

' S r a 162 6 16 Claims. 1

This invention relates to a method of forming crystalline magnesia, or periclase, of high purity and of high density, with decreased effective surface, with the aid of a catalyst which promotes the desired crystallization at much lower temperatures than the art has hitherto been able to employ with magnesia material of like purity.

Magnesium oxide of quite high purity has been very diflicult to prepare in the form of wellcrystallized' piecesjor aggregates of low porosity, heating to temperatures in excess of 2000" C. commo'nlybeing required for satisfactory crystallization of even the technically puregrade. Since such temperatures; are very difilcult to attain in fuel-fired furnaces, and since the resulting porosities are higher than is satisfactory for many purposes, crystalline magnesia of purity better than about 95% M gO is ordinarily prepared by fusion in electric furnaces. Such fusion is a difiijcult process and yields an expensive product which is relatively inert and unsatisfactory for some purposes. For example, unless of extremely high purity, the electrically fused material is difiicult to bond together to form satisfactory high temperature cerami'carticles.

In order to enable crystallization of the magnesia to take place at lower temperatures-, such as those attainable in a rotary kiln-, for example, up to about 1800 C., it has been the practice in the art to add to the magnesia, prior to firing, from to 15 percent of impurities, including silica, lime, alumina and iron oxide. These impurities flux with the magnesia, enabling sintering and crystallization to take placeat tem-. peratures of from about 1550" C. to1800? 0;, de-.- pending upon the amount and kind of additives. In such practice, however, the higher tempera, tures within this range are required to produce a material having low residual shrinkage, that is,.less than about 10% porosity.

Although useful in; enabling the. burning; of magnesia at lower temperatures with, production of good crystallization, the presence. of. these large amounts of impurities in the magnesia is objectionable for. many purposes. When the crystalline magnesia, material is to be, used for ceramic or refractory purposes, the impurities markedly reduce the; over-all ref-ractorinessv of the material, and even. more markedly; lower; its ability to bear load at high temperatures,' as well as its resistance to thermal spalling and to corrosion by acidic materials.

{in object of thisv invention is, tov provide a methqd for forming crystallized magnesia of in:

creased density and of decreased efiecti've sur- 2 face. provide wall-crystallized magnesium oxide. of high purity without resorting to fusion or the addition of fluxes. Another, object is to provide a method of forming crystalline, high purity magnesia at temperatures attainable in fuel fired furnaces, such as rotary kilns, and, if desired, at high production rates. A further object is; to provide a method whereby formation of crystalline magnesia from magnesia-yielding materials proceeds substantially to completion at h lower mp r tu es. a more r p ly han has heretofore; been possible with, the. magnesia of like purity;

According t the present. invention it has now been discovered that the crystallization of magnesia and.- the formation of well-crystallized magnesia, or periclase, from magnesia material which forms, or yields; periclase upon firing ime proved i a. ca a yti mann r b i m ely i in h r w th up to 5- calculated a A 20 in the fired product, of an aluminum compound. Mixtures-of the aluminum substances can be employed. The intimate mixture of the magnesia Starting t r a a d the; a m num c mp und is then fired until. crystallization of the ma nesia is substantially complete. The firing can b pe me n a r ar k ln, at. equ n temperatures, with production of a periclase of desirably low porosity;

I as en n that t e a t of the aluminum compounds described below. yielding p w d ti na ont t; Q 1 n n m.ca cula-ted as AlzQs, in; the; fired magnesia product result in better crystallization of magnesium oxide at a given temperature. or equivalent crystallization at a lower. temperature, when compared with similar. magnesia. fired without such addition, The amount of'the; element present'is calculated as the-sesquioxide in accordance with the ua me q s i expr ss ng. a a e oi r n ory comp und but t s n t. no ctl inwhat state the element exists in the fired product, as it may be present in another oxide form or as. some other compound. uch. a p nal; r t ce S me me e, op im m e t is. obtained with only 0.5%; of the aluminum subst nc calcu A t v In. e al, he more sur th ma nes o neric se yieldi s. mat al; ea er the mp v men obtain d by this process. It is usually'preferred', for the best cryst i n nil 'est no osit eslt add from bou :59; tQ ahout 2-Q.%; ca ulat d. a ltos;

the red b s s. .1.". the alumin m ma er a when this is the sole additive. The aluminum 11-. is also, an object of this invention to substance can be added in combination with small, or catalytic, amounts of other crystallization-promoters, for instance, with chromium compounds, as disclosed in Austin et al., U. S. 2,487,290, or with iron compounds, as disclosed in Austin copending application Ser. No. 101,650 or with both such additives. In such case, a portion of the aluminum material is substituted for by the chromium compound or iron compound, or by both; that is to say, when these substances are added in combination with added aluminum material, the total amount of all these added materials is up to 1.5%, calculated as the sesquioxide, R203, based on the total weight of the fired products. u

The magnesia starting material is a magnesium compound which will form, or yield, periclase,

larly as a solution or as a colloidal suspension. Preferably, the aluminum material is added as a dispersion in water, especially as a solution therein, but any other liquid can be employed. For instance, aluminum bromide can be added as a solution in alcohol, carbon bisulfide or acetone; and, similarly, other aluminum material can be added in solution in a liquid solvent therefor or as a dispersion in a desired liquid, as, for instance, aluminum hydroxide can be added as a suspension in water. Many of the magnesia starting materials contain appreciable amounts of aluminum, generally reported as the sesquioxide, A1203, but it may be present in solid solution in the MgO or it may be poorly dispersed, or both. Whatever the reason, it does not, as is 1 obvious from the results of firing magnesias of crystalline magnesium oxide, upon firing. Such material is employed in finely divided condition,

and it includes, for example, magnesium hydroxide, magnesium carbonate, magnesium basic carbonate, magnesium alcoholate, magnesium sulfate, magnesium chloride, etc. A suitable starting material 'is high-purity natural magnesite or brucite. It is especially advantageous to employ as starting material a precipitated magnesium compound such as magnesium hydroxide, magnesium carbonate or basic carbonate, etc., or cryptocrystalline magnesia. The precipitated compounds'are in suitably finely divided condition. The cryptocrystalline magnesia is magnesium oxide which exists in the form of very small crystals, that 'is, which has not been fired to the form of well-crystallized periclase; in other words, the magnesia crystals are too small or poorly developed to be resolved by the highest power of optical microscope. This is usually known in the art as amorphous or active or caustic magnesia. It'is obtained, for example, by firing magnesium carbonate, basic carbonate, hydroxide, etc. to not over about 1200* C. for not over about 45 minutes, or at a higher temperature "for a shorter time, or at a. lower temperature for a longer time. When these magnesium compounds are so fired, the magnesia formed is not completely shrunken but exists in a cryptocrystalline, or microamorphous, state. When treated according to the present invention, it is improved as to crystal size and porosity. Another suitable starting material is finely divided hydrated magnesia. The magnesia o-r periclase obtained upon firing any of these starting materials preferably contains at least 95% magnesium oxide and less than 2.0% CaO and less than 2.0% SiO'z. Mix- 'tures of these magnesium compounds can be employed. Preferably the magnesium compound starting material is of a particle size to pass through a screen having 100 meshes per linear inch (149 microns diameter), but results are further improved when material passing 200 mesh ('14 microns) is employed.

The aluminum material is a compound of aluminum, such as alumina, aluminum hydroxide, aluminum sulfate, alkali metal aluminates such as sodium or potassium aluminate, aluminum chloride or other halide, aluminum phosphate, etc. Mixtures of the aluminum materials or substances can be employed. The aluminum substance is added in thorough and intimate admixture with the magnesia starting material. This can be effected by employing the aluminum compound also in finely divided condition, preferably passing 200 mesh. However, especially good results are obtained by adding the alumi-{ num material in dispersion in a liquid, 'particu the prior art without separate aluminum additive, have the characteristic effect obtained by means of the present invention. It is to be understood, therefore, that the amounts of aluminum material recited in this specification and vclaims are the amounts added, and are exclusive of any alumina or aluminum compound present in the magnesia starting material itself. For example, where it is stated that aluminum material is added to provide 1.0% of A1203 in the fired product, this amount is added in addition to whatever amount may be present as impurity in the magnesiastartingmaterial, and such provided amount does not mean the total amount of A1203 present in the fired periclase, but only the added amount.

The method of this invention comprises intimately admixing or uniformly interdispersing a finely divided magnesium compound which forms periclase upon firing and up to 1.5%, preferably from about 0.5% to about 1.0%, of aluminum or a compound of aluminum, calculated as A1203 on the fired basis, and firing to form well-crystallized periclase. The aluminum material is employed in a finely divided form, or in solution in .a suitable liquid, or dispersed in a suitable liquid. The admixture is fired at a temperature at which shrinkage occurs, for example, at a temperature .of at least about 1300 C. However, it is an advantage of the present process that periclaseforming admixtures described herein can be fired to crystallization which will remain stable at temperatures of use, at a temperature about 400 C. below that heretofore required for firing magnesia or periclase of such purity.

. By the process of the present invention a denser fired product is obtained as measured in weight per unit volume; and larger crystals of periclase are obtained than when firing the same magnesia or magnesium compound without the additionof the aluminum material as described. The crystals obtained are angular and strong, being approximately equidimensional. The fragments obtained by crushing the larger aggregates are also approximately equidimensional and therefore very suitable for packing well into dense bodies, as in refractory batches. It is a special advantage of this invention that it enables obtaining magnesia, material in the form of grains of low porosity and low residual shrinkage.

The mode of carrying o'ut'the present invention is more clearly demonstrated by the following examples. Example I Magnesium hydroxide is obtained by reacting sea water with calcined dolomite to precipitate MgtQHh, and the precipitated Mg(OH )2 is washed with fresh water and is filtered, Finely pulverized aluminum hydroxide powder is, thor oughl-y admixed with the filter cake. from the above operation, in several batches. In A, no AMOH) sis added; in B, 3.84 grams of A1(OH)3v are admixed with 996.16 grams of the filter cake cor responding to 0.25% A1203 in the fired product) in C, 7.68 grams Al OH 3 are mixed with 992.32 grams filter cake. (0.5% A1203, fired basis) in. D, 15.35 grams A1 OH 3 are mixed with 985.65 grams filter cake (0.5% A1203, fired basis) in D, 15.35 grams Al(OI-I)3' are mixed withv 984.65 grams filter cake (1.0% A1303, fired basis), and in E, 23' grams Al l-l 3 are mixed with 977 grams filter cake (1.5% A1203, fired basis) Each batch is; thoroughly mixed, dried, pressed into briquettes and fired for one-half hour at 1500 C., The porosites of the. fired products are: A, 14.2% B, 10.8%; C, 8.5%; D, 6.6%; and E, 13.0%. By firing a series of similar mixes at 13001C., and. another such series at 1700 C., porosity value curves are obtained which are in general parallel to a. curve obtained by plotting the above porosity values. obtained by firing at 1500 C. The aluminum compound efiects a marked decrease in porosity of the crystalline magnesia, or periclase, obtained upon firing to crystallization equilibrium, when admixed in small amounts, up to 1.5% on the fired basis. After this decrease, with increasing amounts of additive the porosities often tend to rise again or to level off. The grain obtained was hard.

Example II 7 Magnesium hydroxide is obtained as described in Example I, and is divided into several batches. One batch, A, is intimately mixed with 0.296%. A1C13.6H20, corresponding to A1203 on the fired, basis; 0.183% FGCls. 61-120, corresponding to F9203 on the fired basis; and 0.825% CrOa, corresponding to CrzOz on the fired basis. After the whole is thoroughly mixed it is pressed into pellets, and the pellets dried, and fired for one-half hour at 1700 C. In a similar manner, batch B is prepared, employing the three additives as used in A, to contain of each, of the added oxides; Batch C, to contain andBatch D, to contain /2% of each of the added.- oxides. Batch E is a blank and is processedin exactly the same, way, but with no added compounds. The porosities of the fired products are: 14.3%; A, 8.5%; B, 8.5%; C, 8.5%;: D, 8.45%. There is exhibited a sharp decrease in porositywith very low, combined additions. The grain material obtained is extremely hard: and tough, and resistant to impact or abrasion.

Instead of dryin d pressing,v the intimatev admixture of periclase-yielding magnesium compound and aluminum compound can be fed directly to a, rotary kiln and fired to well-crystallized. periclase.

In another variation of this process, particular advantages are obtained by employing a two-stage firing process. In one variant, the starting magnesium compound with the admixed aluminum. compound, with or without other catalytic, crystallization promoters, is calcined to a temperature less than about 1300 C., or preferably" to not over about 1200 C., that is to form a mix-- ture which contains cryptocrystalline magnesia, and without effecting substantially complete, shrinkage; or, in other words, to convert the. magnesium compound of the mixture to cryptocrystalline or active magnesia. Then the. calcinedmixture is comminuted. pressed and fired eta temperature oi at least, 1300? and pr ferably at:- a t mperat re of from 1500 6.. to 17 G1, to form well-crystallized periclase. Inanother variant, the magnesium compound is calcined to form. cryptocrystallme magnesia, and the aluminum. compound, with or without other cataa lytic promoters, is admixed with this. orypto. crystalline magnesia, or with; cryptocrystallme magnesia from any source, and the mixture of magnesia and catalyticpromoters so obtained is comminuted, pressed and. fired as described above, to form well-crystallized Dericlase. A denser and tou her cry t lline product is obtained by pressing oryptocrystallinev magnesia inintimate. admixture with the aluminum compound, and then firing to form the highly crystallized periclase. Such admixture. can be obtained by the. methods described above. This practice pro vides a final periclase of low" porosity, and. of increased strength. better able to withstand tumbling or handling incidental. to use. If de, sired, there. can be mixed with the cryptocrvs talline magnesia-containing material up to of finely divided, well crystallized periclaseand the mass. pressed and fired, preferably at a tern.- perature of at least 1600 C.,. to obtain an. ewecially dens and strong product. The periclase employed in this step. is preferably substantially all of less than 74. microns diameter, and; pres dominantly less. than 44 microns. Advantageously at least 10%, and preferably at; least 50% is less than 10 microns. diameter. The employment of well-crystallized periclase of such partie cle size, in the manner described, provides a fired product. of greater density and which exhibit decreased shrinkage upon firing.

Temperatures other than those shown in. the examples can be employed in firing the mixtures. to form well-crystallized periclase, according'to the invention. The final firing temperature is at least 1300 C. and can be higher. Preferably, a temperature of from about 1500 C. toabout 1750- C. is employed. It is an advantage of the invention, therefore, that the mixes. can be fired to the well-crystallized state in a rotary kiln, or at an equipment. temperature for an equivalent time. 1

The. manner in which the invention functions is' not completely understood, but following is one theory of its operation. When periclaseyielding materials, especially precipitated magnesium compounds, which, upon. firing or heating, yield magnesia. containing at least 95.0% MgO and less than 2.0% CaO and less than 2.0% S102, are so heated as to form crystalline magnesia, very little coalescence or crystal growth occurs and the magnesia crystals obtained are still very finely divided and of extensive surface. As stated above, electric fusion serves to form larger crystals but at high cost, and the addition of fluxing ingredients introduces substantial amounts of impurities which alter the physical and chemical characteristics of the product. The compounds employed herein as crystallizati'on promoters differ in behavior from the flux-'- ing agents in that the optimum amounts of the present compounds are smaller than is the case with fiuxing agents, increasing amounts yield higher porosities in many instances, and increas.-. ing amounts of other impurities which normally act as fiuxing agents, for example, silica, tend to hinder the crystallization-promotihg action of the compounds added according to this. inven-. tion, contrary to. the operation of mix ng ingredients. The effect of adding the compounds of this invention is apparently greater on hi purity magnesia material. The action is considered to be a catalytic effect because small additions of the compounds noted initiate crystallization more rapidly, and produce better crystallization than is obtained with untreated magnesia. The product obtained, which is dense and tough and exhibits high purity and lower residual shrinkage, is useful in many applications, for instance, in refractories, heat exchange media and abrasives.

Percentages given herein are by weight, except in the case of porosity, which is expressed in percent by volume. The porosity is determined by mercury displacement, employing vacuum to remove entrained air.

In conformity with common practice in reporting chemical analyses of refractory materials, in the specification and claims the proportions of the various chemical constituents present in a material are given as though these constituents were present as the simple oxides. Thus, the magnesium constituent is expressedv as magnesium oxide or MgO; the silicon constituent, as S102 or silicon dioxide; the aluminum constituent, as A1203 or aluminum oxide and similarly for other elements reported, although the silica or aluminum oxide and a verysmall portion of the MgO, for example, may be present in combination with each other or with another minor constituent. For example, the term 1.0% by weight of aluminum as, or calculated as, A12 or 1.0% by weight A1203 is intended to mean that a chemical analysis would show the aluminum content as 1.0% expressed as A1203, although actually all of the aluminum might be present as spinel, Mg0.Al203, or in some other combined form.

The term magnesium compound which yields periclase upon firing, or magnesia-yielding compound, or periclase-yielding compound is intended to include cryptocrystalline or microamorphous magnesia and active or caustic or amorphous magnesia, as well as magnesium compounds which decompose upon heating to form magnesia or periclase; such a compound, for instance, being magnesium hydroxide, magnesium carbonate or basic carbonate, magnesium alcoholate, etc. In conformity with the disclosures in U. S. 2,487,290 and Ser. No. 101,650, referred to above, the chromium material should be intimately admixed with the magnesium compound, either in finely divided condition or as a solution, preferably in water, or the like, and the iron compound should be added as a suspension or solution in a liquid, preferably water.

Having now described the invention, what is claimed is:

1. Process for preparing crystalline magnesia which comprises intimately admixing a finely divided magnesium compound which yields periclase upon firing containing at least 95% MgO and less than 2.0% CaO and less than 2.0% Si02 and an aluminum compound, said aluminum compound being added in an amount to provide up to 1.5% aluminum as added calculated as A1203, based on the total Weight of the fired product, and firing said mixture to form high purity periclase.

2. Process as in claim 1 wherein said aluminum compound is added in an amount to provide from 0.5% to 1.0% aluminum, calculated as A1203, based on the total weight of the fired product.

' 3. Process as in claim 1 wherein said aluminum compound is aluminum chloride.

4. Process as in claim 1 wherein said aluminum compound is aluminum hydroxide.

5. Process as in claim 1 wherein the aluminum compound is sodium aluminate.

6. Process as in claim 1 wherein the aluminum compound is admixed in solution in water.

7. Process as in claim 1 wherein the magnesium compound is active magnesia.

8. Process as in claim 1 wherein said mixture is fired at a temperature of at least 1300 C.

9. Process as in claim 1 wherein the aluminum compound is partially replaced by an iron compound dispersed in liquid and the aluminum compound remaining is present in a minimum amount of on the order of about aluminum, calculated as A1203, based on the total weight of fired product.

10. Process as in claim 1 wherein the aluminum compound is partially replaced by an iron compound dispersed in liquid and a chromium compound and the aluminum compound remaining is present in a minimum amount of on the order of about aluminum, calculated as A1203, based on the total weight of fired product.

11. Process for preparing crystalline magnesia which comprises admixing a finely divided precipitated magnesium compound which will yield periclase upon firing containing at least 95% MgO and less than 2.0% CaO and less than 2.0% S102, and a water dispersion of an aluminum compound in an amount which will provide up to 1.5% A1203 as added, based on the total weight of the fired product, pressing, and firing said mixture to form high purity periclase.

12. In the production of crystalline magnesia by a process wherein a magnesium compound which forms periclase upon firing is fired containing at least 95% MgO and less than 2.0% CaO and less than 2.0% Si02 in intimate admixture with a small amount of aluminum compound, the steps comprising pressing an intimate admixture of cryptocrystalline magnesia and up to 1.5% of added aluminum compound calculated as A1203 and based on the total weight of the fired product, and firingv said pressed admixture at a temperature of at least 1300 C. to form well-crystallized high purity periclase.

13. Process as in claim 12 wherein said aluminum compound is partially replaced by an iron compound dispersed in water and a chromium compound and the aluminum compound remaining is present in a minimum amount of on the order of about 6% aluminum, calculated as A1203, based on the total weight of fired product.

14. Process as in claim 12 wherein up to of finely divided well-crystallized periclase, based on the total weight of fired product, is admixed with said calcined mixture.

15. Process for preparing crystalline magnesia which comprises admixing a finely divided precipitated magnesium compound which will yield periclase upon firing containing at least MgO and less than 2.0% CaO and less than 2.0% S102, and a water dispersion of aluminum hydroxide in an amount which will provide up to 1.5% A1203, based on the total weight of the fired product, pressing, and firing said mixture to form high purity periclase.

16. Process for preparing crystalline magnesia wh ch comprises intimately admixing a finely divided magnesium compound which yields peri-.

9 clase upon firing containing at least 95.0% MgO, less than 2.0% CaO and less than 2.0% S102 and at least one aluminum compound chosen from the group consisting of alumina, aluminum hydroxide, aluminum sulfate, sodium aluminate, potassium aluminate, aluminum halide and aluminum phosphate, said aluminum compound being added in an amount to provide up to 1.5% aluminum calculated as A1203, based on the total weight of the fired product, and firing said admixture to form high-purity periclase containin less than 2.0% CaO and less than 2.0% S102.

LESLIE W. AUSTIN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date Pike Aug. 12, 1919 Browne Feb. 24, 1925 Ridgway et a1. Apr. 21, 1942 Chesny Apr. 28, 1942 Heuer Aug. 9, 1949 Pike Aug. 9, 1949 Austin et al Nov. 8, 1949 

1. PROCESS FOR PREPARING CRYSTALLINE MAGNESIA WHICH COMPRISES INTIMATELY ADMIXING A FINELY DIVIDED MAGNESIUM COMPOUND WHICH YIELDS PERICLASE UPON FIRING CONTAINING AT LEAST 95% MGO AND LESS THAN 2.0% CAO AND LESS THAN 2.0% SIO2 AND AN ALUMINUM COMPOUND, SAID ALUMINUM COMPOUND BEING ADDED IN AN AMOUNT TO PROVIDE UP TO 1.5% ALUMINUM AS ADDED CALCULATED AS AL2O3, BASED ON THE TOTAL WEIGHT OF THE FIRED PRODUCT, AND FIRING SAID MIXTURE TO FORM HIGH PURITY PERICLASE. 